Anal. Chem. 1995,67, 280-282
Scanning Laser Photoelectrochemical Microscopy of Reaction Dynamics at a Microelectrode Array Roger S. Hutton* and David E. Williams
Department of Chemistry, University College London, 20 Gordon Street, London, WClH OAJ UK
The photothermal effect in photoelectrochemical microscopy has been used to obtain images of the function of a random array of gold disk microelectrodes. Image resolution of the order 1 pm has been demonstrated. An approximate theory for the image contrast due to the photothermal effect on a reversible electrode reaction at an isolated microdisk electrode is given. In this work we have applied photoelectrochemical microscopy*utilizing the thermal effects associated with a focused light beam2 to investigate the current distribution at a gold microdisk array. The primary aim was to explore the limits of image resolution in this method. The signal arises because the local temperature rise (-1 K) caused by the focused light beam induces a small change in the local current, seen as a small variation on top of the dark current. For the reaction of solution species, the effect is expected to be a maximum at the half-wave potential. Transient effects due to the finite time for the rise and fall of the local temperature and for the relaxation of the local diffusion field are seen. These effects may limit resolution. Images obtained below the limiting current at a larger gold disk electrode2 (250 pm diameter) have indicated that the primary photothermal current response consists of an annulus overlaying the electrode edge. The origin of this contrast has been rationalized by considering the reaction distribution at a disk electrode for a quasireversible reaction below the limiting current.3~~ Several other image features have been identified2besides the primary contrast. Local areas of activity (hot spots) may occur due to enhanced diffusion at submicrometer surface asperities. These simple ideas imply that for a quasi-reversible reaction if the diffusive flux is increased by using a microelectrode the photoelectrochemical signal should be increased. EXPERIMENTAL SECTION
The principles of photoelectrochemical microscopy and the experimental methods used in the present work have been described in detail elsewhere? The microscope was based around a Bio-Rad Microscience Ltd. MRC 600confocal laser microscope, modified to obtain variable line scan speeds. The light source was an argon ion laser (488 and 514 nm, 10 mw). The image shows the change in current caused by movement of the beam ~~
(1) Williams, D. E.; Kucemak. k R J.; Peat, R Electrochim. Acta 1993. 38, 57-69. (2) Hutton. R S.; Williams. D. E.]. Chem. SOC.,Faraday Trans. 1994.90.345347. (3) Baker, D. R; Verbrugge. M. W. J. EZectrochem. SOC.1990, 137, 18321842. (4) Verbrugge, M. W.; Baker, D. RJ. Phys. Chem. 1992.96.4572-4580. (5) Peat, R.; Kucemak. A R J.; Williams, D. E. Electrochim. Acta 1992, 37, 933-942.
280 Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
Figure 1. Specular reflectanceimage of a gold microelectrodearray.
Inset shows closeup.
across the surface. Microelectrode arrays were fabricated by encasing in Korvex (Chemplast Inc. Wayne, NJ) heat-shrink tubing bundles of borosilicate glass sheathed gold wire? The gold wire was -10 pm in diameter, in sheathing -7 pm thick. The wires were secured by heating and coating in epoxy resin. Prior to measurements, arrays were polished with 30, 10,3,1, and 0.3 pm alumina powder water slurries. Analytical grade chemicals were used throughout with triply distilled water. Potentials were measured relative to a saturated calomel electrode (SCE). RESULTS
The geometry and topography of the microelectrode array were examined (Figure 1). The array was found to consist of -50 (10pm diameter) gold wires randomly distributed in a Korvex and glass matrix. The majority of microdisks are located within a central bundle and are separated by -10 pm, corresponding to the borosilicate glass coating thickness. Some remote microdisks are also apparent. Most of the electrodes appear as reflective disks, although some larger and distorted reflective regions are visible. At higher magnifications (see inset) fine contrast is evident, indicating that the gold surfaces are rough. Inspection by scanning electron microscopy demonstrated that the majority of individual microelectrodes lay level to the array surface and exhibit submicrometer surface roughness. Nearly all the microelectrodes formed a good seal with the surrounding glass, a few displaying a crevice around a central gold cylinder. The spatial reactivity across the array was investigated by photoelectrochemically imaging ferrocyanide oxidation (Figure 2). Comparison with the optical image indicates that not all the (6) Made by Chemring Ltd., Portsmouth UK Taylor process: Taylor, G. F. Phys. Rev. 1924.23,655. Taylor, G. F. US. Patent 1793 529, 1931. 0003-2700/95/0367-0280$9.00/0 0 1995 American Chemical Society
Figure 2. Photoelectrochemicalimage of an array electrode at 200 mV versus SCE: 5 m mol dm-3 &Fe(CN)6 (0.1 mol dm-3 K2HP04 0.1 mol dm-3 KH2PO4); laser velocity 3.9 cm s-l; grey scale 100 nA; image 767 x 512 pixels, pixel size 1.27 pm, beam diameter 2 pm; average of 14 frames.
+
0.0
0.1
0.2
0.3
0.4
Potential /V vs. SCE Figure 3. Plot of photocurrent versus potential for various array elements. The lines are according to eq 8. Solid line corresponds to theoretical response for an isolated microelectrodeof diameter 10 mol dm-3 &Fe(CN)e; DO= 1 pm: Er= 0.2 V versus SCE; 1 x x cm2 s-I; A 9 = 10 J K-I m01-l.~
microelectrodes are electrochemically active, particularly electrodes distant from the central bunch. The inactivity is attributed to an electrical break between the electrode surface and rear contact, presumably bending of the wire resulting in loss of electricalcontact. Some image blurring and shadowing is evident, as expected as a consequence of the transient relaxation of the electrode temperature and local diffusion field? However, in Figure 2 additional blurring of the response is visible that is not collinear with the laser spot trajectory and therefore cannot be associated with a transient relaxation of the electrode temperature or diffusion field. This streaking, which is not observed in specular reflectance images, is attributed to heating of the gold below the array surface, a feature related to the orientation of wires to the surface. This interpretation is supported by the observation that, upon rotation, the smearing rotated around with the array. Figure 3 shows a plot of the image pixel intensity versus potential for various electrodes over the array. In acquiring these, data measurements were made at a low laser spot velocity in order to eliminate shadowing and blurring effects, thereby enabling the response at individual microelectrodes to be determined inde-
pendently. Figure 3 confirms that the response was indeed maximal at the half-wave potential and is also different on different elements of the array (as is clearly visible in Figure 2). Figure 4 shows an optical and photoelectrochemical image at higher magnification in order to demonstrate the resolution achievable. First, in contrast to photoelectrochemical microscopy images at larger disk electrodes,2 an enhanced response is not observed at microelectrode edges. Second, contrast across each microdisk is evident, which closely matches specular reflectance images and is consequently attributed to variations in the temperature change due to surface roughness. The spatial resolution is deduced from the separation of the smallest visible features to be -1 pm and in this case is apparently not limited by the spreading of the heated zone on the time scale of the exposure of each pixel. DISCUSSION
In previous work we have interpreted the image contrast in ferrocyanide oxidation in terms of the current change expected for a quasi-reversible reaction
where 6T denotes temperature perturbation, i the dark current density, A& the activation energy for electron transfer, and a the laser spot area. On a microelectrode, the signals were much larger and the edge effects were absent. This result strengthens the conclusion that the edge effects on a macroelectrode were due to the enhancement of the diffusive flux at the electrode edge. The enhancement of response at a microelectrode implies that the discussion is better framed in terms of the photothermal response for an electrochemically reversible reaction. The calculation is based upon the effect of temperature on the reversible electrode potential. Consider a process such as
where the diffusion coefficients for 0 and R are identical and equal to Do. It may be shown7 that the steady state voltammogram of a reversible reaction at an isolated microelectrode is given by
i=
ldiff
1 + exp(nF(E - E,)/RT
(3)
where Er denotes the reversible potential and where idiff, the radial diffusion-controlled limiting current at an inlaid disk microelectrode, is given b y
(Cob is the bulk solution concentration of electroactive species, and d the disk diameter.) For small perturbations the effect of a change in the temperature on the reversible potential E, is simply given by (7)Oldham, K. B. In Microelectrodes: Theory and Applications; Montenegro, M. I., Queiros. M. A. Daschbach, J. L, Eds.; NATO As1 Series 197; Kluwer Academic Publishers: Hingham, MA, 1991; pp 35-50. (8) Saito, Y. Rev. Polarogr. 1968, 15, 178.
Analytical Chemistry, Vol. 67, No. 2,January 75, 7995
281
Figure 4. (a, left) Optical and (b, right) photoelectrochemical images of {he microelectrode array at higher magnification: 5 m mol dm-3 &Fe(CN)6 (0.1 mol dm-3 K2HP04 0.1 mol dm-3 KH2P04); 200 mV versus SCE; 25 mdline; grey scale for photoelectrochemicalimage 100 nA; image 767 x 512 pixels; beam diameter 2 pm.
+
aE,/aT
= aP/aT = AS%F
(5) where Si,,
denotes the current perturbation at E = E,,
where Eo denotes the standard electrode potential and A 9 is the standard entropy change for the electrode reaction. Differentiation of eq 3 and substitution into the relationship
therefore provides the expected photothermal response at an isolated disk electrode, if the whole disk was to be uniformly heated:
Si,
= -idi,#A9/4nF)
ST
(9)
The lines on Figure 3 are plotted according to eq 8, showing consistency of the theory with experimental data. Furthermore, substitution of idfi from eq 4 and of A 9 for the reaction (10 J K-l mol-' 9)into equation 6T 1K, in agreement with the calculated variation for pulsed illumination based upon the absorption coefficient of go1d.1*2 The differing responses for different electrodes in the array have two possible interpretations, either variations in 6T for each element or variation in idiff at each element, the latter caused by the overlap of diffusion fields with adjacent elements. CONCLUSION
wheref= F/m.This theory is an approximation for photothermal microscopy since the effect of the focused light spot is not to heat the whole surface but to impose a local perturbation onto the system resulting in a local perturbation of the diffuse flux of material to the electrode. The approximation would become more accurate as the electrode diameter decreased toward the spot diameter. The perturbation is maximum at E = E,, and its sign and magnitude is dependent on the sign of A 9 . Equation 7 can be recast to allow direct comparison with experiment (9) Weaver, M.J.J. Phys. Chem. 1979.83.1784-1757.
282 Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
Image resolution on the micrometer scale has been demonstrated for scanning laser photoelectrochemical microscopy of a microelectrode array. Nonfunctioning elements could be identified. The current response expected for a reversible reaction as a consequence of the effect of temperature on the reversible electrode potential was consistent with that observed. ACKNOWLEDGMENT This work was funded by the Science and Engineering
Research Council. The authors are grateful to Bio-Rad Microscience Ltd. for technical assistance. Received for review October 19, 1994. Accepted October 28, 1994.@ AC941021C e Abstract published in Advance ACS Abstracts, December 1, 1994.
